Reactive power control of AC transmission lines is desired to control power factor or control currents and voltages. Of course, both inductors and capacitors may be utilized. Such systems are termed in the art a "static VAR." The acronym "VAR" stands for volt-ampere reactive. And a general discussion of reactive power control and actual experiments is contained in Bulletin 13-02 of the "International Conference on High Voltage Electric Systems" dated Aug. 28 through Sept. 3, 1988.
In that bulletin there is a discussion of the use of "switched series capacitors," as illustrated in FIG. 1 of the drawings. Here in series with a transmission line 10 designated X.sub.L with the generator end being 10a and the load end 10b there are inserted series capacitors X.sub.1 through X.sub.n. These are switched by associated series switches S.sub.1 through S.sub.n. These series switches S and capacitors X are in parallel with one another. By selectively inserting one of these series capacitors in the transmission line (illustrated in FIG. 1), this compensates for inductive reactance to a predetermined compensation level. Thus, the reactive impedance of the overall transmission line is controlled, increased or decreased, as desired.
In the system shown in FIG. 1, when the largest compensation level is desired only a single capacitor or series element is connected with the others being switched out of the circuit; thus, the capacitor X.sub.1, for example, in FIG. 1 must be designed for the largest current.
FIG. 2, also prior art, illustrates the practical design of capacitors X.sub.1 where it is actually a bank of series and parallel capacitors. The dashed lines indicate the connection to the other parallel capacitors which are individually switched in and out of the circuit. Other portions of the capacitor module X.sub.1 include the safety bypass switch 11 and a non-linear resistor 12. In one case it has been suggested that the capacitor module might include a single mechanical bypass 13 which can control the percent series compensation capacitance level of the single unit as a one-time adjustment. This was discussed in a paper by E.W. Kimbark entitled "Improvement of System Stability by Switched Series Capacitors" (IEEE Transactions on Power Apparatus and Systems), Vol. PAS-85, Feb. 1966, pp. 180-188. In essence, a single capacitor bank was divided into two sections and a one-time mechanical switch provided for determining percent compensation.
A difficulty with the above scheme of using a bank of parallel capacitor modules is that the installed VAR capacity is excessive. And this is especially true if it is desired to utilize such compensation scheme for automatic and controllable variations in response to the monitoring of a parameter of the transmission line, such as subsynchronous oscillation, etc.